In a closed glass tank which is filled with water to the brim, there are two spheres, one of cork and one of lead. The cork sphere is fixed to the bottom by a thread to prevent it from rising further; the lead sphere hangs from a thread fixed to the top of the tank. The lengths of the threads are such that the lead sphere is hanging just above the cork sphere. Explain what happens from the point of view relative to the tank and from the point of view relative to the Earth, if the tank is now suddenly moved to the right.

           __________________
          |        |         |
          |========|=========|
          |========|=========|
          |========|=========|
          |========O=========|  <----- lead sphere
          |==================|
          |========O=========|  <----- cork sphere
          |========|=========|
          |========|=========|
          |========|=========|
          |________|_________|

(In reply to Agree with Charlie by Kenny M)

Bouyancy forces result from the fact that all fluids expereince increase in pressure as one moves in the fluid towrds the direction of gravity.  If an object immersed in the fluid is less dense than the fluid, the object can float, since the bouyancy force on the object, when completely submerged, exceeds the weight of the object.

This priciple holds whether the fluid is compressible or not, and, in the more general case of an acceleration filed - per Charlie, who btw correctly attributes to Einstein the equivalency of accleration and gravity - the bouynacy force would oopse the net accleration of the bulk fluid, whcih in this problem has both a downward and rightward component

Posted by Kenny M on 2006-01-08 19:13:24